Abstract

Steganography, or communication through covert channels, is desirable when the mere existence of an encrypted message might cause suspicion or provide useful information to eavesdroppers. Text is effective for steganography due to its ubiquity; however, text communication channels do not necessarily provide sufficient redundancy for covert communication. In this paper, the authors propose a novel steganographic embedding scheme for Thai plain text documents that exploits redundancies in the way particular vowel, diacritical, and tonal symbols are composed in TIS-620, the standard Thai character set. This paper provides a Thai text stegosystem following a provably secure construction that guarantees covertness, privacy, and integrity of the hiddentext message under meaningful attacks against computational adversaries. In an experimental evaluation, the authors find that the message embedding scheme allows 203 bytes of embedded hiddentext message per 100KB of covertext on average, and that the document modifications are not readily noticed by observers. The stegosystem is thus a practical and effective secure system for covert communication over Thai plain text channels.

Introduction

Privacy is a major concern for users of public networks such as the Internet. Traditionally, privacy is among the central concerns of cryptography, which achieves private communication through encryption. One problem with encryption, however, is that although it may hide the contents of a message, the mere transmission of an encrypted message may cause suspicion.

As a motivating example, consider a scenario in which a government officer from some country is sent to work in another country for purposes of deepening international relationships between the two countries. Any electronic communication with the guest could easily be monitored by the host country's intelligence agency. If the guest is observed sending or receiving messages in encrypted form, he or she might be suspected of espionage. This could be illegal in some countries and could affect the relationship of mutual trust between the countries.

To avoid such suspicion, information hiding techniques collectively called steganography have received attention for a long time. Steganography dates at least as far back as ancient Greece (Ryder, 2004). In one story, a messenger had a message tattooed onto his shaved head, waited until his hair grew back, and then was sent to deliver the message. Since the secret message written on the messenger's head could not be read except by those who knew what to look for, his mission was successful. In modern times, steganography, which attempts to hide the existence of message transfer, has become a new major concern for the users of public communication networks.

Among the existing practical steganographic schemes, images and video streams are the most popular covertext media, since they contain more redundancy than other types of covertext. However, in some situations, image and video steganography might not be effective, particularly when bandwidth is limited. Text documents are smaller in size than other types of covertext. Moreover, in the real world, people normally do their digital communications through text media such as email messages. Sending and receiving text messages has become normal daily behavior of humans everywhere. Therefore, text is an effective covertext medium for steganography.